[resources/maildir] Reload configuraton on configuration change

Summary:
When user adds new maildir, a new resource gets created, with the default
directory `~/.local/share/local-mail`. Since it's a new resource, with no
proper configuration, an `attemptConfigRestoring()` is called, which changes
it to `~/.local/share/akonadi_maildir_resource#`. It's stored inside
`mSettings->path()`.

Then a dialog appears, where user can choose prefered directory. It gets
written to the config file; then `configurationChanged` gets called.
We call `mSettings->save()`, which overwrites the path provided by user with
the default one (`~/.local/share/akonadi_maildir_resource#`), making it
impossible to create a new maildir anywhere else.

Just use `load()` instead, it makes more sense when configuration was changed.

BUG: 416287
CCBUG: 415245
CCBUG: 415922

Test Plan:
1) Create a new maildir resource pointing to `/tmp/dummy`.
2) The resource gets created, with the name `dummy`. `/tmp/dummy` gets created.
2) Drop some mails into it via KMail. Mails appear inside `/tmp/dummy`

Reviewers: dvratil, mlaurent

Reviewed By: dvratil

Subscribers: wbauer, kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D27905