branch comparison improvements

[Ginxo]

Caused by #44

• add popover for the swap button, something like "Swap left and right branches, remember the order matters here"
• add popover for the number just to the right of the branch selector on the header, something like "The number of different files comparing branch from the left and the right"
• add popover for the Number of Differences from every repository, something like "The number of different files comparing branch from the left and the right"
• solve UX issue for the GAP while data is loading (use loading from redux data.service), it only happens when there are branches to compare information. I would suggest to show a kind of scaffolding occupying same space as the expected final component (selector and so on). Steps to reproduce it
  • Open the webapp
  • wait until the whole page is loaded
  • open browser's developer tools
  • Go to network tab and select "2G Regular" (or even 3G) throttling
  • click on Reload button
  • the selectors do not appear until data is loaded