Use a widget instead of loose javascript code and changes to shiny for tomselect

[oganm]

Current implementation of tomselect used for gene selection relies on code injection into shiny package. It can break with internal changes to shiny and it is harder to read. Two possible solutions are

• Move all code into javascript
• Make a proper shiny widget or an htmltools. Currently working on a widget at https://github.com/oganm/TomSelectWidget

Related to #45 and #43