Versions: allow latest to be a tag

[stsewd]

We were kind of always expecting latest (machine created) to be a branch (the field is even named default_branch!).

But we were allowing both branches and tags to be the default version (latest).

readthedocs.org/readthedocs/projects/forms.py

Lines 272 to 279 in 53e21bb

	versions_choices = self.instance.versions(manager=INTERNAL).filter(
	machine=False).values_list('verbose_name', flat=True)
	self.fields['default_branch'].widget = forms.Select(
	choices=[default_choice] + list(
	zip(versions_choices, versions_choices)
	),
	)

The identifier of a tag is a commit, but users reference latest from the name of the tag, so I'm storing the name instead of the commit in that case. We could do the same for stable to solve #10715.

Closes #10735

[humitos]

Overall it looks good. However, I'm not 100% sure to understand all the small details of this change. Mainly the query where we exclude the latest slug.