[mms] Add support for 'ajaxaction_data' hook (won't be documented in Horde until 5.1+).
[mms] Add the 'nocache' option to Horde_Imap_Client_Base#fetch().
Don't cache fetch results when exporting a mailbox, since there is no guarantee user ever wants to look at that mailbox

Requires the forthcoming Horde_Imap_Client 2.8.0 release to actually
take effect.
Merge branch 'master' into imp_6_1