User Custom Actions Config Page

The following is what I consider to be the easiest way to add script to your SharePoint and SharePoint Online

"Inject JavaScript to any Site or Site Collection via a User Custom Action's ScriptLink property."

2016-03-10 Update

Add Script ID - this enables PnP SharePoint 2013/2016 Responsive UI to be installed by copying 3 files!
Dropped /SiteAssets/ in path, this means you can add deep nested paths into other libraries, or even subsites. ~sitecollection/ + url (before was ~sitecollection/SiteAssets/ + url

Why use User Custom Actions?

This is a great technique because it lets you:

Add JavaScript anywhere - scoped to Site Collection or Site.
Add CSS via JavaScript
You can add more than 1 CSS file
Order them the way you want via Sequence
You can combine this to load your initial JavaScript file which can be a RequireJS setup and then hand off the controls to RequireJS config
Does not modify MasterPage
Works in SharePoint 2010, 2013, 2016 and SharePoint Online
Only need Site Collection permissions to set up - you don't need to have a Farm Solution or Add-In Model. The permission is only required to set up the ScriptLink.
The object model provides a way for an administrator to check all the User Custom Actions attached to any site/site collection, so there's a level of oversight available if you want to check if your customizations are ready for migration.

There are various ways to attach a script via User Custom Actions.

Remote Provisioning (Pattern and Practice) uses it via C# CSOM
PowerShell remote provisioning
Farm Solution can invoke the API
Sandbox Solution can invoke the Client Side Object Model API (*with permission)
Add-In can invoke the CSOM API as well (with Site Collection - Full Control permission)

The unfortunate part is, there's no UI for a power user to add or view ScriptLinks directly. You need to spin up SharePoint Manager or read/update it via PowerShell.

So what do you have here?

I build a simple config page in JavaScript. Then I did a load of work to make sure everything runs from One Page.

Screenshot

I also brand it to look a bit like SharePoint. Just a bit.

Branding

Install

Drop configure-page.aspx into a SiteAssets or SitePages library.
The JavaScript on the page detects and loads some dependencies (jQuery, SP.js etc).
Provided you have site collection permissions, it'll list all existing User Custom Actions
You can specify a filename (including any subfolders like spg/hello.js) and give it a sequence number (default to 1000). Then you can install a Custom Action to Site Collection or Current Web. All via the magic of JavaScript.

Specifying different sequence numbers.

hello.js is sequence 1000. hello1.js is sequence 999.