Further clean-up of Serials interfaces

Further clean-up of Serials interfaces

When trying to come up with good tests, found a few bugs (imagine!).
Some have been nagging for a while now.

1) For 'MFHDDrawn' event, make sure it runs if deleting the last
serials data of any kind for that record (to unpopulate the menus
properly)

2) Make sure global mfhd member (g.mfhd) is cleared in the same case
(i.e. deleting the last serials data for that record)

3) Add a string to better indicate when no MFHD records are available
for 'Legacy Record Entry' drop-down

4) Convert submitted empty drop-down values to 'null' as needed in
distribution editor

5) Mark subscription 'start date' as a required field

6) Make serial virtual records better handle deleted 'legacy record
entries' attached to distributions, and also expose non-deleted
record entries even if in 'use_sdist_only' summary mode

Signed-off-by: Dan Wells <dbw2@calvin.edu>