General UI Optimizations

[swissspidy]

Let's improve the accessibility and the overall UI of the plugin for a better UX.

Related: #30

[timse201]

Think it would be nice to add:

• faq in the footer besides documentation or replace it because it is german only. the faq can be translated on translate.wordpress.org and the link can be changed there to the specific language page. ( Links - FAQ, Support #55 )
• a link to the support forums ( Links - FAQ, Support #55 )
• the current cache size to the settings page, too
• inform the user that saving these settings will flush the cache
• placeholders ( exclusions to variable textareas #53 )
• variable boxes ( exclusions to variable textareas #53 )
• disable expiration and inform about it while switching to hdd + faq ( hide selection for cache expiration - hdd #62 )