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! :))

Fix dependencies, use spaces instead of tabs in .spec file, rename
`imapdomo.spec` to `template.spec`, rename `imapdomo.install` to
`install`, remove unnecessary `override_dh_auto_install`, add URL to
.spec, update Summary, clean up.