Preselect all projects in group coverage analytics

What does this MR do and why?

For #323666 (closed)

Currently, when a user lands on the Repositories Analytics page for a group, to see a list of projects in the "Latest test coverage results" section, they have to select projects from the project dropdown. This MR pre-selects the "All projects" option (which selects up to 100 projects from the group) in the dropdown so that the user will see projects in the list right away, without having to select them. They can then narrow down the list or search for a specific project via the dropdown if they choose to.

Screenshots or screen recordings

before	after
Screen_Recording_2021-10-12_at_11.57.44	Screen_Recording_2021-10-12_at_11.55.31

How to set up and validate locally

1. git checkout 323666-preselect-all-group-coverage-projects
2. create a group
3. add projects to the group that generate coverage results (here are some clone-able coverage-generating projects: 1, 2, 3)
4. run pipelines in one or more of the projects to generate coverage results
5. navigate to the group, then in the sidebar choose Analytics -> Repository
6. in the Latest test coverage results table, the dropdown should default to "All projects selected", and there should be projects displayed in the list

MR acceptance checklist

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