Make GitLab more robust through Snippet Size limits

Goal

Make GitLab search more stable and robust against unintentional or intentional abuses and scenarios where "noisy neighbors" (users, projects, groups) are able to disrupt the snippet functionality by just doing one action.

Background

The Create Teams have been asked via Epic &1737, to proactively find application limits within the Editor code. The Editor Team identified limits in create-stage#40

Solution

Application Limit	Limit size of snippets to X MBs, or limit indexed size to first X MBs
What is the risk of not adopting this new limit?	Causes indexing failures (https://gitlab.com/gitlab-org/gitlab-ee/issues/12111). Can be abused for e.g. hosting files (but this applies to all text fields like e.g. issue descriptions, so more of a general concern)
Estimated Weight	1
Configurable/ Not Configurable	Is Configurable
Are there existing limits?	Yes
Include this in the implementation	Add length validation, figure out what to do with existing snippets that exceed the limit, possibly add a setting for this
Existing Limits	unlimited from the app side / limited by DB field to ~1GB
Existing Limits Documented Properly?	No