* lint: Implement HTML Linting with htmlhint and fix indent.
Implements HTML linting using htmlhint and uses
indentation rules of zulip webapp - 4 spaces.
Creates a separate file .htmlhintrc for the rules, most
of which are defaults. Also, fixes indentation in html
files and adds a missing title in about.html.
* deps: Change versioning of various dependencies to exact versions.
As we prefer to use exact working version
for dependencies, changed the versions to
exact versions.
Fixes#676.
