[bukuserver] Tags-refresh action

[LeXofLeviafan]

fixes #608:

• adding a "Refresh" button to the tags actions panel
• implementing a POST endpoint to reload tags list

P.S. Is tags.html actually used anywhere? I get the feeling that about half of existing templates have been substituted completely by default flask-admin ones. (They seem to be mentioned in the code but these functions don't appear to be actually used anywhere…)

[rachmadaniHaryono]

is tags.html actually used anywhere?

looking at it, no. it is also old code

remove get_tags and update_tags from server.py and tags.html template

those are replaced by ApiTagView

[LeXofLeviafan]

The same seems to apply to search_bookmarks() (in server.py), as well as index.html (never used), bookmarks.html (used only in search_bookmarks()), and base.html (used only in index.html and bookmarks.html).

[rachmadaniHaryono]

those can be removed on other pr

[jarun]

Thank you!