Make "Misc Donation" an editable field

[armahillo]

In #70 , @seanmarcia suggested ( #70 (comment) )

Maybe we should make it part of the Mark 2 stretch goal of making it an editable field.

Current function is that it's a single drop-down list, populated from an array of Strings, found in /app/models/donation.rb.

This would need to probably refactor sources to be a separate model (so that "other" is an option) or it could add an additional "misc_comment" column to the donations table. Would probably want to override the accessor/mutator for the source field to accommodate the "Misc Donation" data? (like, when it's reading that field, if it says "Misc Donation" as the source, it concatenates the "misc_comment" field to the end, and when it's assigned, it splits that apart.)