Allow users to edit Version's identifier and/or slug

[humitos]

This is an initial POC to test the minimal implementation that we discussed in https://github.com/readthedocs/meta/discussions/147#discussioncomment-11397703.

The main goals here are:

• Keep all the user/original values separated from the real Version
• Do not update edited Version with values from VCS
• Keep syncing code working properly
• Make the minimal changes to prove this idea is possible

Re-point a slug to a different identifier

1. Edit the version
   

2. Build the version

• Note the name of the version is the correct (update-conda-startup)
• Note the git clone command clones the correct branch
• Note the name of the branch is not correct. We can't use verbose_name there, we should use identifier if it's overriden by the user

3. Serve the new identifier under the previous slug
   

Change the slug of a version

1. Edit the version
   

2. Build the version
   

3. Serve the version under the new slug
   

[humitos]

I think this PR proves what we want to do here is possible. We will need to research a little more to find out edge cases and handle them as well (I saw there are special cases for LATEST and STABLE versions that I didn't touch on purpose).

Besides, we will need to think more about the UI/UX since there are places where verbose_name won't be accurate anymore and we will need to use the override.user_identifier instead. Overall, the workflow I followed looks good, but it will definitely need some polish.