Add clarifications to group repository analytics page

What does this MR do and why?

For #349826 (closed)

This MR makes some UX improvements:

• Adds an explanatory paragraph under the page title
• Adds a last updated date next to the Test Code Coverage title
• Adds explanatory popovers to each single stat
• Adds a help icon with popover next to the Latest test coverage results title
• Moves the Download historic test coverage data (.csv) button into the Latest test coverage results card header, make the button and dropdown flex-wrap on narrower viewports
• Splits the timespan description text (Last 30 days) off from the graph card title

And does some refactoring:

• Moves the page title from haml to vue
• Moves the Test Code Coverage title into the summary component
• Passes the group name (groupName) to the vue app for use in some of the explanatory text
• Fixes some vue warnings about undefined tooltip data by setting it to null by default
• Extracts the i18n text for most of this page into one constants file
• Switches to GlTableLite for the coverage table because it doesn't have any sorting or filtering (yet)
• Make coverage table header buttons wrap better
• Aligns the project select dropdown contents with the right side of the dropdown trigger

Screenshots or screen recordings

before	after

popovers
Screen_Recording_2022-05-10_at_21.18.17

How to set up and validate locally

1. have a GitLab Premium licence or higher
2. git checkout 349826-group-repo-analytics-clarifications
3. find or create a group, and within that group create projects that have coverage jobs
   tip: clone this test project and run pipelines using a variable called COVERAGE for an easy way to generate coverage results
4. navigate to the group, in the sidebar find Analytics => Repository
5. you should see aggregated coverage data from all the group's projects, though it might take a bit of time to show up

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.