Fix inconsistent font size for markdown preview on new PR view

[CirnoT]

We use same method for new issue form and issue view, but was missing from new PR view making it one place where markdown preview is inconsistent in font size.

[zeripath]

$ grep -Rh font-size web_src/less | sed -e 's/^ *//' | sort | uniq                           [09:41:35]
font-size: 100%;
font-size: 1.1em;
font-size: 11px;
font-size: 1.25em;
font-size: 128px;
font-size: 1.2em;
font-size: 12px;
font-size: 13px;
font-size: 1.3rem;
font-size: 1.4em;
font-size: 14px;
font-size: 1.5em;
font-size: 15px;
font-size: 1.5rem;
font-size: 16px;
font-size: 1.6rem;
font-size: 1.75em;
font-size: 1.7em;
font-size: 1em;
font-size: 1rem;
font-size: 20px;
font-size: 21px;
font-size: 2.25em;
font-size: 2.3rem;
font-size: 2em;
font-size: 3.5em;
font-size: 36px;
font-size: 3em;
font-size: 5.5em;
font-size: .5em;
font-size: .75em;
font-size: .7em;
font-size: 85%;
font-size: .85714286rem;
font-size: .8em;
font-size: .8rem;
font-size: .92857143rem;
font-size: .9rem;
font-size: inherit;
#git-graph-container li .time {color:#999999;font-size:80%}

Ugh

[zeripath]

Thanks @CirnoT

In order to reduce the delays for merging I wonder if we could try to bunch some of these styling fixes (for at least the backports.)

[zeripath]

make lg-tm work