Make GitLab more robust through limiting the size of the wiki page

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 wiki functionality by just doing one action.

Background

The Create Teams have been asked via Epic &1737, to proactively find application limits within the Knowledge code. The Knowledge Team identified limits in create-stage#39.

Solution

Application Limit	Size of individual wiki page
What is the risk of not adopting this new limit?	Wiki pages will be slow to load. Long-running ruby process (markdown) can be denial of service attack surface
Estimated Weight	1
Configurable/ Not Configurable	Is Configurable
Are there existing limits?	Yes/No
Include this in the implementation	Should be configurable (critical for any existing users with large pages).