Capture "other role" details during signup

What does this MR do?

Use text field to capture further details of Other role type during signup

Related to #255170 (closed)

Testing

Enable the feature flag

Feature.enable(:user_other_role_details)

Screenshots

user-other-role2-2020-12-01_11.46.18

Migration output

$ VERBOSE=true bin/rails db:migrate:up VERSION=20201127141433
== 20201127141433 AddOtherRoleToUserDetails: migrating ========================
-- column_exists?(:user_details, :other_role)
   -> 0.0019s
-- add_column(:user_details, :other_role, :text)
   -> 0.0039s
-- transaction_open?()
   -> 0.0000s
-- current_schema()
   -> 0.0002s
-- execute("ALTER TABLE user_details\nADD CONSTRAINT check_b132136b01\nCHECK ( char_length(other_role) <= 100 )\nNOT VALID;\n")
   -> 0.0034s
-- current_schema()
   -> 0.0002s
-- execute("SET statement_timeout TO 0")
   -> 0.0002s
-- execute("ALTER TABLE user_details VALIDATE CONSTRAINT check_b132136b01;")
   -> 0.0078s
-- execute("RESET ALL")
   -> 0.0002s
== 20201127141433 AddOtherRoleToUserDetails: migrated (0.0379s) ===============

$ VERBOSE=true bin/rails db:migrate:down VERSION=20201127141433
== 20201127141433 AddOtherRoleToUserDetails: reverting ========================
-- column_exists?(:user_details, :other_role)
   -> 0.0018s
-- remove_column(:user_details, :other_role)
   -> 0.0008s
== 20201127141433 AddOtherRoleToUserDetails: reverted (0.0090s) ===============

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team