fix: #5255 prevent shortcode conflict with reserved system routes

[StephDriver]

closes #5255

[StephDriver]

Notes:

1. this doesn't only apply to journals, but to repos.
2. on creation journal and repo codes need to be cross checked as if two have the same name it would also cause a conflict
3. other than these codes, the reserved routes would be part of the codebase and so can be a static list with a unittest to prevent drift, rather than needing to be dynamically built each time they are needed for validation.
4. Baking in accessibility alongside this - WCAG 3.3.1, 3.3.3, 3.3.6 around errors.

[StephDriver]

The first commit here sets the respository shortcode to be unique. I have included a safeguard to warn if non-unique values are found on migration. Ordinarily this would be a breaking change, however in this particular case I cannot see how duplicate values could have gone unnoticed. They would have caused problems accessing the different repos. So all this is doing is bringing the repository shortcode into line with teh journal shortcode which already has 'is_unique.

This makes the validation against reserved routes simpler.

@ajrbyers are you happy with this approach?

[StephDriver]

pausing this work as 1.9 testing is more urgent. This is in a state that someone else could probably pick it up if need be. I think the solution is mostly there, but it needs full testing.

work to do:

• review failing tests. Some are also failing on master.
• manually test