[PORT] Make MOST shields able to be worn on your back (sprites included)

[Anui-ElXx]

About the PR

Ported the ability wear shields on your back and sprites from Gaby-Station/Gaby-Station#150
I've also changed the riot-bullet-shield worn-sprite to match the Omu's ballistic shield
Riot shield worn-sprite has been changed a bit because I thought it looked ugly.

Why / Balance

It looks nice and it's a cool mechanic. Far too often I would leave my shield somewhere and forget where I left it, now we can just put it on our back.
Also, if you are using a shield, you are probably using either a melee or a gun that doesn't need to be wielded, in both cases you can just put the weapon inside your backpack

Technical details

• added wearing feature to shields.yml; 6 different shields are now able to be worn on your back (ballistic, riot, clockwork, laser, makeshift and buckler)
• added sprites for the ballistic shield (sprite added to _goobsation/objects/shields/riotbulletshield.rsi folder because omu uses goob's sprite for the riot-bullet shield)
• meta.json changed to make the new sprites work
• added sprites for all the other 5 shields (sprites added to objects/weapons/melee/shields.rsi)
• meta.json changed to make the new sprites work
• added wearing feature to shields.yml; decreased the active sound volume just a bit (_goobstation/entities/shields) - for Blueshield's shield (also applies to deathsquad shield and all other greatshields)
• meta.json changed to make the new sprites work

*MAYBE IMPORTANT: * the ballistic shield appears invisible when held in hand.
It was working fine, but after closing the client, editing some shield-worn sprites, and restarting, the held sprites stopped loading.

I tried switching to master branch and re-cloning the repo fresh (no changes made) and it's still invisible.
Asked on Discord; some players said they never saw held sprites for it, so this might be an old bug.
Can't find anything in my changes that would cause this."

Media

• loud sound warning, turn your volume down a bit

shields.2.1.mp4

• The first picture (front) shows the shield off, that is why the sprite is not appearing. My bad

Requirements

• I have read and am following the Pull Request and Changelog Guidelines.
• I have added media to this PR or it does not require an ingame showcase.

Breaking changes

Changelog

[github-actions]

RSI Diff Bot; head commit 25b4a65 merging into 5bf019b
This PR makes changes to 1 or more RSIs. Here is a summary of all changes:

Resources/Textures/Objects/Weapons/Melee/shields.rsi

State	Old	New	Status
buckler-backpack			Added
metal-backpack			Added
ratvarian-backpack			Added
riot-backpack			Added
riot-laser-backpack			Added

Resources/Textures/_Goobstation/Objects/Shields/blueshieldshield.rsi

State	Old	New	Status
equipped-BACKPACK-on			Added
equipped-BACKPACK			Added

Resources/Textures/_Goobstation/Objects/Shields/riotbulletshield.rsi

State	Old	New	Status
riot-bullet-backpack			Added

Edit: diff updated after 25b4a65

[CliveOcelot]

If your willing, it would be nice to do this for the Greatshield as well

[Anui-ElXx]

If your willing, it would be nice to do this for the Greatshield as well

Done

[CliveOcelot]

Needs comments

[CliveOcelot]

looks good to me