use okhttp client for caching (instead of glide)

[David-Development]

This MR disables Glides Caching and uses OkHttp's caching instead. The reason is that Glide does not care about ETags and Last-Modified Headers. Once a URL is in the cache it stays there. See: bumptech/glide#1847

Fixes #1247

Changes:

• Glide cache is now only 10mb as it is supposed to be used only for favicons
• Default DiskCacheStrategy is now NONE - only favicons override this setting
• OkHttp has a Cache now (which has the size of what's configured in the app settings - by default 500mb)

TODO:

• Offline caching of images does not currently work (see todo in code)
• Injecting images into articles from cache currently does not work (as the cache check only checks the glide cache - not the okhttp cache)

[David-Development]

@tobiasKaminsky FYI - I saw that you were involved in the dicussion in bumptech/glide#1847. I think by using the OkHttp Client we can get around the issues with ETags / Last-Modfied without having to initiate a separate http request. But then again... this requires OkHttp which you are not using in the files app yet, eh?

[tobiasKaminsky]

Nice idea!
On Files we slowly switch to okhttp3, so this can then be a way to go.

[Unpublished]

This needs to stay as is see #1315