refactor(heatmap): added builds fetch on demand

[gis727]

Why this PR?

Completes #243. Fixes #144.

What does this PR do?

This PR:

• Updates the heat map in order to:
  • Fetch batches only at initialization.
  • Fetch builds for a specific day only when the user clicks on the corresponding batch.
  • The builds are cached. So, clicking on a batch that has already been viewed will not initiate a new request.
• Updates the repository page in order to:
  • Show the heat map and the tests list only when they are both ready.

Can this PR break anything in master?

Yes if #243 hasn't been merged into master.

Can those changes be staged?

Yes. On request.

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