More fixes for supporting Relative URL

Make flash messages fixed and sticked to bottom

Improvements to English text

Fix the link to the project branches

Ref: https://github.com/gitlabhq/gitlabhq/commit/e6159b8725f99af78f446f8d33fa0e52b7780430
Ref: https://github.com/gitlabhq/gitlabhq/pull/3204
Ref: https://github.com/gitlabhq/gitlabhq/issues/1228

Add Rails' variable in application.rb to support relative url

This variable is used by assets compilation and other modules.

Note that user needs to change application.rb too

Restrict session cookie to the relative path if set.

Ref: https://github.com/gitlabhq/gitlabhq/commit/2c2f1e31856a4decdae469974f5bea8245316f7e

Fix Update attachment_uploader.rb bug with relative URL

See: https://github.com/gitlabhq/gitlabhq/commit/161afda3fa4fca58f396e9c3acbd72bc14490ace

Fix Wall relative bug with attachement files (javascript)

Thanks!

Definitely sounds better like this.

Summary:

 - Mostly very small things involving use of "a", "the" etc.
 - "post-receive" hooks are usually written with the hyphen in the name .
 - I changed the Web Hooks page to have a title consistent with all the other
   pages in the settings area.
 - More consistent usage of full-stops in lists.

These are just a few very particular fixes for small grammatical issues,
intended to polish the reading experience.