(Bug 4826) Remember the previous security level after spellcheck

* for reasons related to changing minsecurity, we rebuild the security
 level dropdown with JS. We go back to the dropdown's displayed
 value only if it the user had already explicitly selected that level,
 rather than us having adjusted it because of minsecurity.

  See SHA: d1b7b5fa for a detailed breakdown of this case.

  Because we wait until the user has explicitly chosen a security level
 from the dropdown, this means that the value of the dropdown on load
 is ignored. This is not an issue when making a new entry
 (minsecurity), and not an issue when editing (that code path is not
 triggered on edit), but is an issue when spellchecking (the initial
 value *has* been explicitly selected by the user).

  This is much longer than the actual code, but the solution then is to
 mark the form as having been spell-checked, and then trust that
 initial value as being one the user chose deliberately.

* also removes a couple TODOs re: spellcheck which are no longer
 relevant since spellcheck is handled for those cases