Fix downloads button positioning and interactivity

[emerick]

Resolves brave/brave-browser#42924

This PR hides the "managed footnote" on brave://downloads, since our version of the downloads page doesn't have a static area to display the footnote (unlike the upstream page). For the same reason, we add some margin-right to the "Clear All" button so it doesn't overlap with teh scrollbar when the list of download items is long.

Submitter Checklist:

• I confirm that no security/privacy review is needed and no other type of reviews are needed, or that I have requested them
• There is a ticket for my issue
• Used Github auto-closing keywords in the PR description above
• Wrote a good PR/commit description
• Squashed any review feedback or "fixup" commits before merge, so that history is a record of what happened in the repo, not your PR
• Added appropriate labels (QA/Yes or QA/No; release-notes/include or release-notes/exclude; OS/...) to the associated issue
• Checked the PR locally:
  • npm run test -- brave_browser_tests, npm run test -- brave_unit_tests wiki
  • npm run presubmit wiki, npm run gn_check, npm run tslint
• Ran git rebase master (if needed)

Reviewer Checklist:

• A security review is not needed, or a link to one is included in the PR description
• New files have MPL-2.0 license header
• Adequate test coverage exists to prevent regressions
• Major classes, functions and non-trivial code blocks are well-commented
• Changes in component dependencies are properly reflected in gn
• Code follows the style guide
• Test plan is specified in PR before merging

After-merge Checklist:

• The associated issue milestone is set to the smallest version that the
  changes has landed on
• All relevant documentation has been updated, for instance:
  • https://github.com/brave/brave-browser/wiki/Deviations-from-Chromium-(features-we-disable-or-remove)
  • https://github.com/brave/brave-browser/wiki/Proxy-redirected-URLs
  • https://github.com/brave/brave-browser/wiki/Fingerprinting-Protections
  • https://github.com/brave/brave-browser/wiki/Brave%E2%80%99s-Use-of-Referral-Codes
  • https://github.com/brave/brave-browser/wiki/Web-Compatibility-Exceptions-in-Brave
  • https://github.com/brave/brave-browser/wiki/QA-Guide
  • https://github.com/brave/brave-browser/wiki/P3A

Test Plan:

• Enable a feature in the group policy to make this a "managed" Brave installation (see https://support.brave.com/hc/en-us/articles/360039248271-Group-Policy)
• Visit brave://settings
• Confirm that "managed footnote" is shown at top of page
• Visit brave://downloads
• Confirm that "managed footnote" is not shown at top of page
• Download a few files
• Confirm that "Clear all" button works
• Download enough files that a scrollbar appears on brave://downloads
• Confirm that "Clear all" button does not overlap with scrollbar
• Confirm that "Clear all" button still works
• Visit brave://settings
• Confirm that "managed footnote" is shown at top of page

[github-actions]

The security team is monitoring all repositories for certain keywords. This PR includes the word(s) "policy" and so security team members have been added as reviewers to take a look.

No need to request a full security review at this stage, the security team will take a look shortly and either clear the label or request more information/changes.

Notifications have already been sent, but if this is blocking your merge feel free to reach out directly to the security team on Slack so that we can expedite this check.

[mkarolin]

chromium_src LGTM

[brave-builds]

A Storybook has been deployed to preview UI for the latest push

[emerick]

@thypon @kdenhartog This PR just mentions the word "policy" as the change slightly tweaks the UI for the downloads page for managed users, which is set via group policy. There are no changes beyond CSS.

[thypon]

approved

[brave-builds]

Released in v1.75.147