deleting duplicate tag name on server
removing tag name on empty text
handling empty tag name and ctrl_cmd command
various fixes, cleanups, and improvments of tagged versions

part 1/2: UI side
for @amol-jore

editor_tab.js
get rid of extra tag arguments that weren't needed
rename tag_notebook -> tag_version
keep editor.tag_version pure of nodes, which are a UI detail
treat empty string also as remove tag
tag was never actually set in options nor should it be: always read it from history

notebook_title.js
call show_history because editor.tag_version shouldn't
store the node with the version tagging function
rename ctrl_cmd -> fork_and_rename
don't modify global editable_opts, make new object for tag version options
use separate maps tag2version and version2tag

it's unlikely that they would overlap but it's just cleaner to keep
the two types separate
fix function call that i broke removing tag argument
use version2tag in rcloud.load.notebook
more fixes, cleanups, and improvments of tagged versions

for @amol-jore

check whether the version had a tag before, and if so, remove its tag2version
user tag2version, version2tag inner functions to generate keys for clarity
Merge branch 'pr/925' into develop

Conflicts:
	htdocs/editor_tab.js
	htdocs/js/rcloud_bundle.min.js