LaTeX environment for dealing with references

This environment is called "reference". We currently use the comment
environment from the verbatim package to ignore anything in between
\begin{reference} and \end{reference} when running latex.

Coding style. We expect this environment to be used as follows

\begin{lemma}
\label{lemma-ABC}
\begin{reference} \cite[page 13]{DEF} \end{reference}
....
\end{lemma}

Or in case there are multiple references for a single result or some
explanation is warranted, we use it like so

\begin{lemma}
\label{lemma-ABC}
\begin{reference}
The first half of this lemma can be found in \cite[page 13]{DEF} and the
second half in \cite[III, Lemma 14.2.3]{EGA}
\end{reference}
....
\end{lemma}
Add a reference for Nakayama's lemma
Add references for Yoneda's lemma
Short explanation of references to the literature in documentation/rules
Do not allow \begin{reference} ... \end{reference} on one line for now
Small change in the rules regarding nested environments

This is silly, because I just realized that we can also have the enumerate
environment inside a lemma. etc. But I guess this is correct with regards to
the Stacks project specific enviroments
Reference for definition birational morphism of schemes
Merge branch 'references'

The minimal necessary infrastructure in
stacks-tools and stacks-website is available
so we merge this now. This will allow us to
add references as we add new material.