Create "bulk term action" form if multiple terms selected while reading

[jzohrab]

e.g, if I've shift-clicked many terms, then it doesn't make sense to show the form for the first term:

The window on the right should really be blank if multiple terms are selected ... but more useful would be to have some kind of "bulk action" form:

• set status (should default to common status if all are the same, otherwise shouldn't force all to be changed??? - might need a checkbox saying "update status", but that looks weird)
• add tag
• remove tag (pre-populated with common tag if one exists)

Adding/removing tags shouldn't fail if adding dup, removing missing, etc etc etc.

[jzohrab]

Done in PR, merged into develop.

[jzohrab]

Launched in 3.7.0