Add ability to manually disable / enable offline Exchange cache

[caomhanach]

There seem to be ongoing issues with performance when offline caching is enabled (see for example #437 and #507).

I do not understand why the ability to disable this feature is not available to users:

1. The offline cache is enabled by default at startup (mivExchangeCalendar.js:664)
2. The "Cache" checkbox on the EWS Calendar and Mail settings -> Offline Caching dialog is disabled and greyed out.
3. The "exchange-cache" checkbox on the Edit Calendar dialog is hidden completely.

This commit enables both checkboxes, and makes their labels consistent (""Enable Offline Exchange Cache").

I can report a huge increase in performance when the offline cache is disabled. Thunderbird was virtually unusable with EWS enabled (symptoms like those described in #437 and #507), but with the offline cache disabled it works quite well. True, having calendar events visible while offline would be nice, but it's not essential for me, whereas a working Thunderbird is. And users should at least have the option to enable and disable the cache at will.

[Langenscheiss]

Can I advertize this request please. Apart from performance, offline caching constantly produces false ...(Busy) entries. Those are a huge nuisance. Either fix the (Busy) calendar entries and improve performance, or give the user the ability to turn off offline caching.