Add check for Content-Type when downloading tracker list

[userwiths]

Closes #13061.

Description

Some users might get confused around platforms like github and their raw links. By mistake such user has entered the HTML page link and each HTML line was taken as a tracker.
In order to fix this issue, we add a check on the Content-Type header of the response. We accept text/plain and show a descriptive error message otherwise.

[glassez]

Some users might get confused around platforms like github and their raw links. By mistake such user has entered the HTML page link and each HTML line was taken as a tracker.
In order to fix this issue, we add a check on the Content-Type header of the response. We accept text/plain and show a descriptive error message otherwise.

The plain text may also contain arbitrary/inappropriate data. Shouldn't the lines themselves be validated instead?

[userwiths]

I thought that the text area was meant to be used in order to "preview" the url's a user was about to add.
But I agree that we could limit their chances of entering gibberish accidentally.
Latest commit adds a simple check to see if the text looks like a url.

[stalkerok]

Doesn't work, incorrect list added without a problem.
Working.

But it looks like you need to add checking here as well.

[stalkerok]

Tested!