Turn tag checkboxes code into a simple loop

This makes the code much cleaner and also reuses the strings in the
dtag_names array. This means the tag names are all lower case everywhere,
but that seems like an acceptable trade-off.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Suggested-by: Henrik Brautaset Aronsen <subsurface@henrik.synth.no>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Only show tags checkboxes if all edited dives have same tags

This is trivially true if there is only one dive.

With this change the logic which tags apply after editing is much easier
to understand.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Add four more tags

Student (training), Instructor (teaching), Photo and Video

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>