[resources/ews] Save password to wallet

Summary:
Seems like the password entered via the UI actually never gets saved anywhere.
Just do it explicitly.

BUG: 393002
CCBUG: 390798
CCBUG: 402780
CCBUG: 414789

Test Plan:
1) Try to setup EWS account using autodiscovery, using Username/Password auth
2) Set Username, Password, hit "Try connect" -> it works fine
3) Hit "OK" -> observe "Authentication failure" resource error
4) Browse `akonadi-ews` via `KWalletManager` -> it's empty
5) Apply the patch, repeat 1-4 - authentication succeeds, password entry inside wallet appears

Reviewers: dvratil, nowicki

Reviewed By: dvratil

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D27813