Normalize message/envelope tags in the Vuex store

[ChristophWurst]

As discovered in #4830 we have to restructure the way we store tags in the Vuex store. Right now we use a deep structure on the envelope objects but it turns out we should very likely normalize the data so that there is an indexed object of tags in the Vuex store and envelopes reference those tags by their id. This is also the way we store accounts, mailboxes, envelopes and messages.

We have to check all usages of tagEnvelope. In contrast to flagEnvelope the value here isn't a boolean but an id that represents the tag.

This will enable us to work on #4807 and #4806 because then tags only exist once and when you change color, the update will be visible for all envelopes simultaneously.

[ChristophWurst]

Related to this change (but not sure if we want to do it in one step) is to load a user's tags with the page, just like we load the json with the accounts. Then we really only have to send around IDs when a tag is assigned and so on. So we'll possible also change

mail/lib/Controller/MessagesController.php

Line 663 in 1910e08

public function setTag(int $id, string $imapLabel): JSONResponse {

so that the browser sends an ID instead of the IMAP label. 👀 @miaulalala