global speaker renaming

[JohannesGarm]

Global speaker renaming through a list under media display in fulltext editor.

• Original list of speakers from workers preserved under generated_speaker field under caption.
• Speaker is updated by entering a new name in place of the existing in the sidebar and either pressing enter or causing the field to loose focus.
• New speaker field to add new speakers (must be assigned via the drop down on the individual caption)
• Prune button to remove names not present on any captions.
• Error notification if a name that is identical to an existing name is submitted.

Issue description:

#53