V2 validate improvements and fixes

[rmouritzen-splunk]

Changes:

• By default, warnings are no longer created for missing recommended attributes. This can be enabled with a new missing_recommended=true query parameter to the /api/v2/validate and /api/v2/validate_bundle APIs.
• Mismatches on regular expression constraints on attribute types are now warnings instead of errors. (This is easier to deal with now that missing recommended attributes no longer create a flood of warnings -- at least by default.)
• Fix bug validating attributes enabled by profile.
• Fix validation error message for deprecated attributes.

Unrelated changes:

• Change deprecated use of <%# for comments in Elixir HTML templates to <%--.
• Update dependencies.