Verify phone number format in trial form

What does this MR do?

Verify phone number format in trial form

• The phone data isn't stored in gitlab, but sent to CustomersDot and Salesforce via an API call
• This first iteration HTML5 input validation on the frontend of gitlab to give better formatted phone numbers

Related to #326452 (closed)

Validity

	Validity
+1 202 555 0156	Valid
202 555 01 56	Valid
202 555-01-56	Valid
(202) 555 01 56	Invalid - Parenthesis are disallowed
(202) aaA=1`	Any letters or non-allowed characters invalid

Screenshots

First form	Second form

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• 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