Version: introduce display_name field for sorting and display

[stsewd]

What's the problem this feature will solve?

We are allowing to change the slug of a version, that basically changes how the URL looks like, but the old name (verbose_name) would still be used in some places. We are also using the slug for sorting, but I think that's a bug (#11182). Slugs also have limitations of the characters that can be used.

Describe the solution you'd like

Introduce a new field called display_name that would be used for sorting and to show it to users in most places. display_name will default to the verbose_name of the version. verbose_name should be unique per project, but it could be more flexible about the allowed characters.

Alternative solutions

Another solution would be to change the versbose_name, but that breaks our modeling of mapping each branch/tag to an RTD version.

Additional context

#11930