Update page objects to handle mobile layout

What does this MR do?

Part of gitlab-com/www-gitlab-com#11567 (closed) gitlab-com/www-gitlab-com#12192 (closed)

Adds :mobile tag.

Updates current qa page objects to handle mobile browser navigation/layouts for mobile browser testing for the first 3 tests:

• qa/specs/features/browser_ui/1_manage/login/log_in_spec.rb
• qa/specs/features/browser_ui/2_plan/issue/create_issue_spec.rb:12
• qa/specs/features/browser_ui/2_plan/issue/create_issue_spec.rb:22

Documentation is in a separate MR gitlab-qa!731 (merged)

Screenshots or Screencasts (strongly suggested)

iphone12creatanissue

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are 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.
• This change is backwards compatible across updates, or this does not apply.

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.