feature #28919 [DX][WebProfilerBundle] Add Pretty Print functionality for Request Content (SamFleming)
feature #28919 [DX][WebProfilerBundle] Add Pretty Print functionality for Request Content (SamFleming)

This PR was squashed before being merged into the 4.3-dev branch (closes #28919).

Discussion
----------

[DX][WebProfilerBundle] Add Pretty Print functionality for Request Content

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        | n/a ?

## Why?
Quite often when attempting to debug issues with JSON requests sent to a Symfony API, I use the Web Profiler to check the request content. More often than not the request content isn't easily readable (99% of the time it's all stuck on a single line and impossible to read). I always find myself copying + pasting the content into a random online tool to have it "pretty-print" the JSON.

Usually this isn't an issue, but can be annoying when offline. There's also the security issue of sending entire JSON payloads to a third-party server just for formatting 😳. Alternatively, maybe developers  copy+paste into their chosen editors and this PR is all a waste of time — I hope not 😛.

## How?
This PR adds "Pretty-Print" JSON functionality straight into the profiler.

We can use `collector.requestheaders` to detect if the request was JSON and conditionally show the Pretty Print button.

When the button is clicked, we format the JSON from the "Request Content" card.

## What does it look like?
Before:
![without-pretty-print](https://user-images.githubusercontent.com/573318/47180751-36b0ce00-d319-11e8-86ed-eb0d78ebcbe3.png)

After:
![pretty](https://user-images.githubusercontent.com/573318/47180763-3c0e1880-d319-11e8-995d-eba565aad827.png)

Non-JSON Requests (unchanged):
![non-json-request](https://user-images.githubusercontent.com/573318/47181080-03227380-d31a-11e8-8cf2-e8b2e8c1a21d.png)

## Things to consider

- Is `JSON.stringify(JSON.parse(content));` the safest, most efficient way to do this?
- Should the "Pretty Print" button be in-line next to the "Request Content" header? I couldn't find a pattern for this sort of thing elsewhere in the profiler.
- Do people want JSON formatted with 4 spaces, would 2 spaces be preferred? Should this be a configuration option stored in localStorage (such as the light/dark theme configuration)?
- Should this be a toggle? E.g. click to pretty print, then click to undo

## Future Improvements

Depending on how this is received it could be extended to support formatting different request content-types (e.g. XML formatting) — I assume.

## Progress

- [x] Gather feedback and decide where to perform the pretty-print: [server-side, or client-side](https://github.com/symfony/symfony/pull/28919#issuecomment-431508361).
*It was decided server-side would be better.*

Commits
-------

9f85103151 [DX][WebProfilerBundle] Add Pretty Print functionality for Request Content

symfony/twig-bundle,symfony/twig-bundle