Support PlantUML blocks in reStructuredText

What does this MR do?

Adds support for PlantUML blocks in the following format:

.. plantuml::

   Bob -> Alice: hello
   Alice -> Bob: Go Away

Based on the Markdown approach in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/8588, the real work is done by Banzai::Filter::PlantumlFilter.

This development is sponsored by Siemens (/cc @bufferoverflow)

Are there points in the code the reviewer needs to double check?

• We could consider using uml:: instead of plantuml:: to match sphinxcontrib-plantuml, I decided to use the latter since we don't support the additional options from sphinxcontrib
• All <pre> tags now also contain a <code> tag, this looks the same on GitLab but might break layout if this gem is used outside of it

Ruby 2.0 build is failing, is this a concern or should I remove it from .gitlab-ci.yml?

you can remove, we don't support ruby 2.0 anymore, and we suggest 2.3 for GitLab.

@brodock ok thanks, removed in 7d3ff66

changed the description

resolved all discussions

changed milestone to %10.1

assigned to @brodock

resolved all discussions

assigned to @smcgivern

@smcgivern can you review this as well?

changed title from Support PlantUML blocks to Support PlantUML blocks in reStructuredText

@smcgivern is out for the week. @DouweM can you take this one please?

assigned to @DouweM

merged

@toupeira Merged and released as v1.6.2! Can you create an MR to pull this into gitlab-ce?

@DouweM thanks! MR is submitted at https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/14395