Alias can't be reused

[Entkenntnis]

1. Create a taxonomy, e.g. schätzen

2. Something goes wrong, so you remove your entry and rename it, e.g. schätzen-verkackt

3. You create a new taxonomy with the same name - now the link looks like: https://de.serlo.org/mathe/deutschland/serlo-lehrplan/5.-klasse/nat%C3%BCrliche-zahlen/sch%C3%A4tzen/-148332

4. The old link is still bound to the entry created in (1): https://de.serlo.org/mathe/deutschland/serlo-lehrplan/5.-klasse/nat%C3%BCrliche-zahlen/sch%C3%A4tzen

Issue

I would like to reuse the original link when the existing taxonomy is trashed

[inyono]

Proposal:

• Aliases can be marked as purgeable
• When checking if an alias is free, you may also use one that is purgeable.
• When an entity gets trashed, mark the alias as purgeable. When it gets restored, remove the purgeable flag if the alias is still in use by the same entity. If not, create a new alias.
• When generating a new alias for an entity (i.e. after renaming), mark the previous alias as purgeable.

[inyono]

Resolved by #540 (needs some more testing before it goes to production).