FCustomTags: rework and svelte-fy

[candela97]

1. Custom tags were stored by guide id in local storage, which were easily lost, so we now fetch and parse the guide page (when editing own guides) to get enabled tags, and only store recently enabled tags in local storage for use when creating new guides.
2. Custom tags depended on the feature that enables multi-selected languages (otherwise the tag inputs won't get checked correctly), and only worked because the feature array happened to be in the right order. This moves that feature into custom tags since it doesn't really make sense without it (extra language tags will be lost when you edit the guide after saving).

TODO Remove old local storage key.

[tfedor]

You use new LocalStorage key, but I don't see a migration from the old one to new one. Am I just missing it?