front-end: add eslint-plugin-html as dependency as introduced in .eslintrc.js in 4d36432bf705

Usage example:
hg up -cr.
sed -i -e 's/\${[^{}]*\({[^{}]*}[^{}]*\)*}/""/g' -e 's/%\(if\|else\|endif\|for\|endfor\)\>.*//g' -e 's/##.*//g' $(hg loc 'kallithea/templates/**.html')
vim kallithea/templates/pullrequests/pullrequest.html +139 # blank out the multi line 'var url = ${}'
( cd kallithea/front-end/ && node_modules/.bin/eslint $(hg loc 'kallithea/templates/**.html')) | tee l
hg up -Cr.

( sed -n 's/^function \([^(]*\).*/\1/gp' kallithea/public/js/base.js; sed -n 's/.* var \([^ ]*\) =.*/\1/p' kallithea/templates/base/root.html; echo pyroutes; echo CodeMirror; echo Select2; echo BranchRenderer;)|while read x; do echo $x; sed -i "/error .$x. is not defined /d" l; done; cat l