Landing page for in-app message surveys

This MR provides a landing page for in-app message surveys. When filling out a survey, users are directed to this page and shown a 'Thank you' message. The results are then passed on to the data warehouse for analyzation via Snowplow on the server side.

Because this page needs to be available on GitLab.com from self-hosted instances, authentication is disabled.

Snowplow expects the following URL parameters when invoking this page from a survey:

parameter	type	required
survey_id	number	true
response	text	true
instance_id	number	false
user_id	number	false
email	text	false
name	text	false
username	text	false

The URL could look like this:

https://gitlab.com/-/survey_responses?survey_id={{survey_id}}&response={{response}}

Snowplow then sends a self describing event to the data warehouse. The event uses this schema: iglu:com.gitlab/survey_response/jsonschema/1-0-0

Screenshot:

Related to: gitlab-org/growth/engineering#2605 (closed)

Design: gitlab-org/growth/team-tasks#51 (closed)

Closes: gitlab-org/growth/team-tasks#50 (closed)