Add substitutions for common URLs

Signed-off-by: Roberto Rosario <roberto.rosario@mayan-edms.com>

docs/chapters/development/code_structure.txt

@@ -174,7 +174,7 @@ Steps to deploy a development version
 
 .. code-block:: bash
 
-    $ git clone https://gitlab.com/mayan-edms/mayan-edms.git
+    $ git clone |SOURCE_CODE_GIT|
     $ cd mayan-edms
     $ git checkout <Corresponding branch>
     $ virtualenv venv

docs/chapters/development/contributing.txt

@@ -8,11 +8,11 @@ If you have a feature request, suggestion or bug report, please open a new
 issue on the `GitLab issue tracker`_. To submit patches, please send a merge
 request on GitLab_.
 
-.. _GitLab: https://gitlab.com/mayan-edms/mayan-edms/
-.. _`GitLab issue tracker`: https://gitlab.com/mayan-edms/mayan-edms/issues
+.. _GitLab: |SOURCE_CODE_REPOSITORY|
+.. _`GitLab issue tracker`: |SOURCE_CODE_ISSUES|
 
 
 Contributing changes
 ====================
 Follow the latest contributing guidelines outlined here:
-https://gitlab.com/mayan-edms/mayan-edms/blob/master/CONTRIBUTING.md
+|SOURCE_CODE_REPOSITORY|blob/master/CONTRIBUTING.md

docs/chapters/development/source_control.txt

@@ -6,7 +6,7 @@ Mayan EDMS source is controlled with Git_.
 
 The project is publicly accessible, hosted and can be cloned from **GitLab** using::
 
-    $ git clone https://gitlab.com/mayan-edms/mayan-edms.git
+    $ git clone |SOURCE_CODE_GIT|
 
 
 Git branch structure
@@ -59,7 +59,7 @@ Special branches:
 Each release is tagged separately using annotated Git tags.
 
 When submitting patches, please place your code in its own ``feature/`` branch
-prior to opening a Merge Request on GitLab_.
+prior to opening a Merge Request on |SOURCE_CODE_ISSUES|.
 
 .. _Git: http://git-scm.org
 .. _`Successful Git Branching Model`: http://nvie.com/posts/a-successful-git-branching-model/
@@ -97,6 +97,3 @@ Commit messages
    Use::
 
        Document: Fix typo in label description
-
-
-.. _GitLab: https://gitlab.com/mayan-edms/mayan-edms/