Tag Management UI #1773

Describe the feature you'd like
I would like a single user interface where I can manage tags and tag values with the ability to see a mapping of where tags are (listing of shelves, books, chapters, and pages).

Describe the benefits this feature would bring to BookStack users
Tags are very easy to add and the live suggestions as you type are great, however, as a project ages, it is quite likely that the tags will get out of control unless there is an easy way to manage them and clean them up periodically. It would be great to be able to go to one place in the UI and be able to both see what tags are associated with what entities (shelves, books, etc.)

Additional context
The essential functionality requires no more than a new UI that simply reverses the normal mapping, which would be this:
[Shelf | Notebook | Chapter | Page] --> [tag1:tag-value1,tag2:tag-value2]
to this:
tag1:tag-value1 --> [Notebook1, Page21, Page23, Chapter3]

A nice to have feature down the road would be to include some data cleaning utilities such as what is included in tools like OpenRefine (similar strings, semantic similarity, misspellings, etc.)...But under no circumstances, should the latter feature be construed as necessary for an initial capability.