Refactor Service Desk's bot user

What does this MR do?

This MR makes adding user bot accounts easier by introducing the enum User.bot_type (with support_bot as only value for now) and stop using users.support_bot boolean database column. We also replace specific support_bot? checks with the more generic bot? where it makes sense.

After this MR we will be able to add more bot accounts (such as GitLab Alert Bot) more easily. See https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10460

No need to

• add a boolean database column e.g. alert_bot
• extend existing support_bot? checks with || alert_bot? in several places

Just

• extend the enum bot_type by e.g. alert_bot
• add new alert_bot? if needed

CE backport https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/26671

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/10159

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer