Thumbnails hardening

[wkloucek]

expose THUMBNAILS_MAX_CONCURRENT_REQUESTS and THUMBNAILS_MAX_INPUT_IMAGE_FILE_SIZE as a configuration when available

[wkloucek]

THUMBNAILS_MAX_CONCURRENT_REQUESTS was also back ported to stable-5.0 -> owncloud/ocis#9252

[wkloucek]

@dschmidt is this something you could have a look at, too?

[dschmidt]

If it's just adding those settings to the thumbnails service, I can probably manage to do that, yeah

[wkloucek]

@d7oc During #654, I looked at it and oCIS 5.0.6 supports now THUMBNAILS_MAX_CONCURRENT_REQUESTS but not THUMBNAILS_MAX_INPUT_IMAGE_FILE_SIZE, so we could only port half of #642. Should I do it anyways?

[d7oc]

Better some protection than nothing I would say.