Select from assigned issues list

What does this MR do?

Primarily:

• Adds list of assigned issues for the user to select
• Adds ability to filter this list
• Retains ability to enter issue URL

Additionally:

• improves some tests

This MR builds on styling/layout work done in !55 (merged).

Demo

Demo video showing filtering, and search by URL:

Note: invalid form feedback left padding is slightly too large in video. See screenshot below for actual, fixed version

mvc-2-styling

Video demo pre-2020-07-10

MVC-2-assigned-issues

Screenshots

Note: placeholder in screenshots out of date. Placeholder is actually "Filter by issue number, title, or paste an issue URL"

Selected issue

Filtering list of issues

When issue URL entered, but invalid

Access Token screen (after slight style changes)

Closes #4 (closed), #27 (closed)

Checklist

• Generated new gitlab_figma_plugin_dist.zip (npm run pkg) file if changes have been made to src/
• Added tests for new functionality