enhancement: Instead of an input for filtering repos, show a dropdown

[AhsanAyaz]

Is your feature request related to a problem? Please describe.
Right now, we show an input for filtering repositories (tabs) if the user has different pull requests opened for different repositories. Nobody likes to type the repo name in the input.

Describe the solution you'd like
Instead of the input, we should show a dropdown containing the potential repositories to select from.

Additional context
By default, the first repository should be selected.
The selected repository also should be persisted using chrome storage.