Add script for generating release notes draft

A script for generating release notes from MR descriptions. It relies on having both a clone of the repository and access to the GitLab API.

Questions/answers:

1. Is LbRelease/scripts the right place?

• A default .md template lives in LbRelease/data. I've copied the one from Hlt, but I don't mind changing it.

2. Where should templates live? Probably in ProjectSys/doc and not in LbScripts?

• The default project template in Project/ReleaseNotes/ will take precedence.

3. Can we get gitlab and jinja2 without prepending lb-run --ext pytools LCG/latest?

• gen_release_notes is the wrapper script and the action is in LbRelease/python/LbRelease/GenReleaseNotes.py

4. git, git_o and getOutput were copied from git-lb-push, but should probably be shared. How?

• Separated out these commands in a new module LbRelease/python/LbRelease/GitTools.py

To test:

# Clone and build LbScripts
cd $TMPDIR
git clone --recursive ssh://git@gitlab.cern.ch:7999/lhcb-core/LbScripts.git
cd LbScripts
git checkout rmatev-release-notes
cd LbScriptsSys/cmt; cmt br cmt make; cd -
cd LbPlatformUtils/cmt; cmt br cmt make; cd -

# LbLogin
$TMPDIR/LbScripts/InstallArea/scripts/LbLogin.sh -m /cvmfs/lhcb.cern.ch/lib

git clone ssh://git@gitlab.cern.ch:7999/lhcb/Hlt.git
cd Hlt
gen_release_notes v26r6 v27r0

LbRelease/data/release_notes_template.md

+{% set used = [] -%}
+{% macro section(labels) -%}
+{% for mr in select_mrs(merge_requests, labels, used) %}
+- {{mr.title}}, !{{mr.iid}} (@{{mr.author.username}}) {{find_tasks(mr)}}  
+  {{mr.description|mdindent(2)}}
+{% endfor %}
+{%- endmacro %}
+
+{{date}} {{project}} {{version}}
+========================================
+
+Short release description
+----------------------------------------
+
+Based on ...
+This version is released on ... branch.
+
+### Line developments
+{{ section(['line']) }}
+
+### New features
+{{ section(['new feature']) }}
+
+### Enhancements
+{{ section(['enhancement']) }}
+
+### Bug fixes
+{{ section(['bug fix']) }}
+
+### Cleanup and testing
+{{ section(['cleanup', 'testing']) }}
+
+### Other
+{{ section([[]]) }}