Support editing Jenkinsfile when it's a multi-branch Pipeline

## Background
The multi-branch Pipeline feature of Jenkins supports running Pipeline dynamically against the specific branches of a git repository. Users do not need to create many Pipelines by themself. Jenkins will load Jenkinsfile directly from the git repository. So, users need to maintain the Jenkinsfile in the git repository. That means they cannot edit the Jenkinsfile from Jenkins UI.

Many users of this project hope that they could edit the Jenkinsfile directly on the UI.

## Proposal
In order to make it to be easier for users. We need to provide the following things:

* APIs for loading, saving Jenkinsfile from a git repository
* UI pages for the editing actions

## References
* [Video about what multi-branch pipeline is](https://www.bilibili.com/video/BV1fp4y1r7Dd?p=23&vd_source=ca1041974e9b44975cc4d5355ad7001f) (in Chinese)

/kind feature

Start by reviewing how the project currently handles multi-branch Pipelines and Jenkinsfiles from a git repository, then map the existing API and UI entry points. Done means users can load and save a Jenkinsfile through APIs and edit it through UI pages.

Written by the indexing model from the issue text.