Create issue_selector component

What does this MR do?

Part of work towards #4 (closed)

Primary changes:

• Create Issue and IssueSelector vue components
• IssueSelector handles the selection of an Issue (and will eventually contain the Issue list for users to select from)

Secondary changes:

• replace as much css as possible with bootstrap/gitlab ui utility classes
• rename accessToken.query.graphql => access_token.query.graphql
• Change dimensions of the plugin window (to make it bigger)
• Add webpack config to import gitlab ui variables to sass files/Vue components

Demo

📹 MVC1-3

Checklist

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

Closes #21 (closed)