lint: Implement HTML Linting with htmlhint and fix indent.

* 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.

.htmlhintrc

@@ -0,0 +1,25 @@
+{
+  "tagname-lowercase": true,
+  "attr-lowercase": true,
+  "attr-value-double-quotes": true,
+  "attr-value-not-empty": false,
+  "attr-no-duplication": true,
+  "doctype-first": true,
+  "tag-pair": true,
+  "empty-tag-not-self-closed": true,
+  "spec-char-escape": true,
+  "id-unique": true,
+  "src-not-empty": true,
+  "title-require": true,
+  "alt-require": false,
+  "doctype-html5": true,
+  "id-class-value": "dash",
+  "style-disabled": false,
+  "inline-style-disabled": false,
+  "inline-script-disabled": false,
+  "space-tab-mixed-disabled": "space4",
+  "id-class-ad-disabled": false,
+  "href-abs-or-rel": false,
+  "attr-unsafe-chars": true,
+  "head-script-disabled": true
+}