sage.misc.latex_macros: do not evaluate anything on startup. See trac #13504
Reinstate 'is_html_markup'.
Fix for sage #13678, allow tab completion of matrix constructor.
Merge remote-tracking branches '@jhpalmieri/mathmode_v2' and '@jhpalmieri/latex-macros'; commit '4696154' from @robertwb

A batch of changes for bumping the sagenb version in Sage
Update installation README

The version of sagenb shipped with recent versions of Sage is now not
ancient, so the specialized instructions on trac ticket #13121 are no
longer necessary.