Replace bootstrap Dropdown to gitlab-ui Dropdown

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR will solve: #348606 (closed)

1. bootstrap legacy dropdown is substituted with GlDropdown
2. Static dropdown uses default version of dropdown
3. Dropdown with search and async data loading is replaced with 'with search version'
4. GlLoadingIcon is used explicitly because built-it loading prop for dropdown doesn't look similar to required design
5. Legacy dropdown is removed with according test (it is not used anywhere else)
6. Async data loading for template list happens only when dropdown opens, only when show event is used, not toggle.

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1. Go to project files /root/$PROJECT/
2. Select any file (.gitlab-ci.yml for better example)
3. Click Edit in Web IDE.

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