Tag Settings Menu Autosave

[ErisDS]

Note that there was some work done towards this in PR #4500.

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All of the fields in the tag settings menu (TSM) should autosave themselves, in the same way as this works on the post settings menu.

When editing or creating a tag, save should be triggered on focus out from the text inputs - but the model should only be saved if the value is not blank, and has changed from it's previous value.

Saving should cause nprogress to fire.

The auto-saving behaviour should still allow the values to get reset when moving between the TSM for different tags, or a new tag.

The save button can be removed.

[naz]

At the moment the tag is prevented from being saved whenever Tag Name is not provided. This creates a bit misleading experience, as someone could skip the tag name field and fill out the slug/description fields, which populates general tags list with a new entry. But whenever the page is refreshed all entries without name are lost. Maybe we should provide some kind of warning that name field is required, or maybe fill it out automatically ?