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/

docs/chapters/docker/building.txt

@@ -6,7 +6,7 @@ Building the image
 
 Clone the repository with::
 
-    git clone https://gitlab.com/mayan-edms/mayan-edms.git
+    git clone |SOURCE_CODE_GIT|
 
 Change to the directory of the cloned repository::
 

docs/chapters/docker/nightly.txt

@@ -7,6 +7,6 @@ produces a resulting Docker image. These are build automatically and their
 stability is not guaranteed. They should never be used in production.
 If you want to try out the Docker images the development uses or want a sneak
 peek at the new features being worked on checkout the container registry at:
-https://gitlab.com/mayan-edms/mayan-edms/container_registry
+|SOURCE_CODE_REPOSITORY|container_registry