doc: Update guide on adding custom content blocks
Document escape_texts

Fixes #743
Merge pull request #741 from mawuli-ypa/documentation-on-blocks

doc: Update guide on adding custom content blocks
core: add #media_identify_extension{} notification.
Add original_filename and extension to #media_identify_file{} notification.
core: ensure that #media_identify_extension{} mime and preferred_extension are binaries.
This makes pattern matching easier.
mod_base: let contoller_page ignore query args and language on page_path check.
This also adds a new query arg  which contains the list of path-parts used for checking the dispatch rules.
Issue #746
mod_base: also let controller_page append qs on page_path redirect.
Fixes #746
core: prevent image-url checksum if only mediaclass defined.
This fixes a problem where a checksum was generated if an undefined width/height was specified, typically in a {% media %}