Compare results from different projects

[arthur-flam]

Is your feature request related to a problem? Please describe.
When working with subprojects, users often want to compare their results versus a different project. For instance, if a subproject is the v1 of a project, and another is the v2, users will want to select v1 as a reference.

Describe alternatives you've considered
So far, users get around the issue by running by either:

• exporting results and comparing with a third-part tool: it's annoying and those 3rd party tools often lack features...
• re-running what they want to compare to as part of their project: it require care to do right and in the end it's still confusing.

TODO

• Review the text field that selects a commit: maybe use an icon or a "select" button that hint it's possible to hover and select a different project in a meny (BTW currently we show the available milestones on hover, it's hard to discover. Let's change this too).
• Offer a select menu with all the related subprojects
• Change state.selected so that it's possible to specify a project... Keep in mind it should still be possible to switch new/ref.
• Milestones should save non-default projects, and the non-default project should appear in the milestone selection menu.
• The backend/api/export_to_folder.py has to do the same matching as the web application, so it could require some tweaking... Same goes for qaboard/optimization.py when optimizing improvement versus a reference.

[arthur-flam]

Done in the last release