A few changes to the ACP
- Remove cachedir from Database & Paths page (fix #2347)
- Do not allow changing the Cache settings page if Settings.php is not writable (would not have any effect otherwise)

Signed-off-by: Yoshi2889 <rick.2889@gmail.com>
Merge pull request #2355 from Yoshi2889/cachefixes

A few changes to the ACP