This patch fixes mentions not being send when multiple sections were added
in between sections.
Since we only want to send mentions when userlinks and signature are present
in the same section a new method was added extracting sections and the related
content from an addition. The results are checked whether a section content
contains a signature and might be relevant for mentions.
Bug: T141863
Change-Id: I434c664552bbadbeef6e897e20703e813f5a4c52
