Easier initiation of Gerrit editing from Gitiles

[Ilmari Lauhakangas <ilmari.lauhakangas@xxxxxxxxxxxxxxx>]

A while ago I created a Gitiles improvement with the help of more 
experienced hackers (my first time using Java). It adds an [edit] link 
to file views when you are browsing files at a branch. Clicking this 
link will immediately create a new patch in Gerrit (a warning dialog 
would be nice and I will look into this later).

I have updated our Gerrit editing wiki article which is based around 
Help, but can of course be applied to any of our repos: 
https://wiki.documentfoundation.org/Documentation/GerritEditing#Inline_Editing_of_Help_pages

I made another improvement to Gitiles which Guilhem kindly applied to 
our instance while it's not yet in an official release: the ability to 
view the source of Markdown files had disappeared 6 years ago and I 
restored it. While viewing something like 
https://git.libreoffice.org/core/+/master/README.md you will now see 
"source log blame" links at the bottom. This also allows easy quick 
edits to the content of our Hugo-based sites such as 
https://git.libreoffice.org/infra/documentfoundationorg/ and 
https://git.libreoffice.org/infra/libreofficeorg/

Ilmari