Use connectedCallback to initialize GlEmoji

What does this MR do?

Issue was discovered in !43900 (comment 425023068):

Bug

Element attributes may not be available during execution of the constructor. This causes problems, as data attributes e.g. emoji name may not be available when we need them. Race conditions may be present which means emojis may not render. See MDN for more info.

Fix

This MR uses the connectedCallback, which will ensure the element has been mounted to the DOM before we initialize the emoji.

Screenshots

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