#408. Fixed error with no enabled post types, added validation for some text inputs.
#408. Merge with last changes.
#408. Backmerge with master.
#408. Fixed content preview length.
Added description for advanced settings page

Added description for advanced settings page
Updated wording for advanced settings page

Updated wording for advanced settings page
#408 Split preview excerpt parameters from teaser content section
Merge branch 'master' into feature/408_add_capability_to_gain_unlimited_post_access
Merge branch 'feature/408_add_capability_to_gain_unlimited_post_access' of github.com:laterpay/laterpay-wordpress-plugin into feature/408_add_capability_to_gain_unlimited_post_access
#408 Fixed install and uninstall of preview excerpt settings
Merge branch 'feature/408_add_capability_to_gain_unlimited_post_access'