jQuery & IE-compatible Staticman integration

[VincentTam]

Hi, thanks for your like to my recent PR to beautiful-jekyll. However, from the discussion in the issue that it addresses, there's a better approach.

Description

1. Change back to jQuery slim version and apply IE & jQuery slim compatible Staticman JS daattali/beautiful-jekyll#782 for backward compatibility.
2. For using Staticman v3 with GitHub, GitHub App is the recommended way of authorization.

Analysis

Run the following command.

git diff $(git log --grep="add staticman" --pretty="format:%h")^1...sm3xhr

to view the difference between the commit before the one titled "add staticman" bee63b16^1 and my sm3xhr branch.: fbdd5b7...VincentTam:sm3xhr

Observe that the files changed in this PR under the paths _layouts/ and _includes/ aren't shown in the files changed in the above linked git diff. That means this PR is restoring the slim jQuery version.

[CLAassistant]

All committers have signed the CLA.