link_to_gfm, think twice [Meta]

Description

link_to_gfm has been a good wrap helper of link_to(gfm()), but cost of this helper is too high by two aspects:

• high computing resource (due to use of Banzai.render and Nokogiri::HTML::DocumentFragment.parse)
• error-prune (See link_to_gfm on gitlab-org/gitlab-ce Issues)

This is a proposal to define a simple guideline to use the link_to_gfm helper.

Proposal

• A single call of link_to_gfm is recommended only if GFM is effective in that context.
  • Consider to use link_to when GFM is not necessary.
• Calling link_to_gfm in a loop is avoided without special reasons.
  • Use link_to almost always.

Current situation

path (in /app/views)	1st argument of link_to_gfm	controller#action	1 or N	issues/MRs	status
/projects/blame/show.html.haml	commit.title (updated by !12900 (merged)) truncate(commit.title, length: 35)	Projects::BlameController#show	N	- (cf. #35163 (closed)) (introduced in eb06dd79)	-
/projects/branches/_commit.html.haml	commit.title	Projects::BranchesController#index, Projects::TagsController#index, Projects::TagsController#show	N	-	-
/projects/commits/_commit.html.haml	commit.title	Projects::CommitsController#show etc.	N or 1	- (cf. #34509 (closed))	-
/projects/commits/_inline_commit.html.haml	commit.title	Projects::CompareController#show	N	-	-
/projects/deployments/_commit.html.haml	commit_title	Projects::EnvironmentsController#show	N	-	-
/projects/tree/_tree_commit_column.html.haml	commit.full_title	Projects::RefsController#logs_tree.json	N	-	-
/shared/projects/_project.html.haml	project.commit.title	Projects::ForksController#index	N	#35742 (closed) !13173 (closed)	❌
/shared/milestones/_issuable.html.haml	issuable.title	Projects::MilestonesController#show, Groups::MilestonesController#show, Explorer::MilestonesController#show	N	#34289 (closed) !12444 (merged)	✅
/projects/issues/_merge_requests.html.haml	merge_request.title	Projects::IssuesController#show	N	!12451 (merged)	✅
/shared/milestones/_milestone.html.haml	milestone.title	Projects::MilestonesController#index, Groups::MilestonesController#index, Explorer::MilestonesController#index	N	!12451 (merged)	✅

Links / references

This was found when working on !12444 (merged) and !12451 (merged).