Allow admins to configure URL aliases

This allows to map plugin screens into the Gerrit URL namespace, e.g.:

  [urlAlias "MyPluginScreen"]
    match = /myscreen/(.*)
    token = /x/myplugin/myscreen/$1

This can also be used to replace Gerrit screens with plugin screens,
e.g. to replace change screen:

  [urlAlias "MyChangeScreen"]
    match = /c/(.*)
    token = /x/myplugin/c/$1

Defining URL aliases is similar to defining comment links. This is why
a similar representation in the gerrit.config was chosen.

Change-Id: Ie4a3b69703629051f7627eeb0c479dfc964f27ae
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>

Documentation/config-gerrit.txt

@@ -3767,6 +3767,46 @@ on the server. One or more groups can be set.
 If no groups are added, any user will be allowed to execute
 'upload-pack' on the server.
 
+[[urlAlias]]
+=== Section urlAlias
+
+URL aliases define regular expressions for URL tokens that are mapped
+to target URL tokens.
+
+Each URL alias must be specified in its own subsection. The subsection
+name should be a descriptive name. It must be unique, but is not
+interpreted in any way.
+
+The URL aliases are applied in no particular order. The first matching
+URL alias is used and further matches are ignored.
+
+URL aliases can be used to map plugin screens into the Gerrit URL
+namespace, or to replace Gerrit screens by plugin screens.
+
+Example:
+
+----
+[urlAlias "MyPluginScreen"]
+  match = /myscreen/(.*)
+  token = /x/myplugin/myscreen/$1
+[urlAlias "MyChangeScreen"]
+  match = /c/(.*)
+  token = /x/myplugin/c/$1
+----
+
+[[urlAlias.match]]urlAlias.match::
++
+A regular expression for a URL token.
++
+The matched URL token is replaced by `urlAlias.token`.
+
+[[urlAlias.token]]urlAlias.token::
++
+The target URL token.
++
+It can contain placeholders for the groups matched by the
+`urlAlias.match` regular expression: `$1` for the first matched group,
+`$2` for the second matched group, etc.
 
 [[submodule]]
 === Section submodule