Several changes regarding packaging, namespacing and handler API:

 *  Utility function module is now called 'imapdomo', not 'common'.

 *  The module behaves as one, instead of being just a global-modifying
    include.

 *  Lots of cleanups as advised by luacheck (local declarations,
    removal of unused variables...)

 *  Handlers now have to access utility API via `require` and provide
    accout configuration in new file mailboxes.lua, which has to return
    table.  Accessing mailboxes is via utility function.  (This sounds
    complicated but actually is not.  Believe me! :))