Add ability to attach merge request

Note to reviewer

• API calls are very slow, please monitor the network tab to make sure they are still loading (See https://gitlab.com/gitlab-org/gitlab-ce/issues/32042#note_33258115 for more details about API slowness)
• Trello allows you to create views for custom content but for list content, you have to use their API to display them
• Trello's list content API does not have a good debounce setup, so we are not doing search queries on the GitLab API even though that was an option in the beginning
• Project list retrieved from API is sorted based on last updated. Will need to check with @victorwu whether that is desired or not. Our projects are quite active and I have seen different projects jump to the top of the list.

What does this MR do?

• Adds the ability to save your auth token and api url - Closes #22 (closed)
• Automatically add the GitLab logo for attachments that are from gitlab.com - Closes #21 (closed)
• Adds the GitLab card button - Closes #2 (closed)
• Card button has the ability to search for a project and a merge request - Closes #4 (closed), #5 (closed)
• Card button has the ability to attach a merge request to the Trello card - Closes #6 (closed)
• Contains the general UI flow of this trello power up - Closes #1 (closed)

What does not work?

• Attach Issue list item button does not work when you click the GitLab card button #20 (closed)
• Authorize with GitLab.com button does not work at the moment #15
• No error messages/catch blocks at the moment #19 (closed)
• No view for Edit Power-Up Settings #14 (closed)