css/custom.css: correct hangouts icon coordinates

[plowsof]

replaces / includes #2307 *Add discord to hangouts

opening the spritesheet in GIMP to obtain pixel correct values:

• Pre coordinaudit

Icon	Default XY	Hover XY	Size	pxl spacing below	between
Facebook	128 0	176 0	43x43	5	5
Twitter	128 48	176 48	43x43	5	5
Reddit	128 96	176 96	43x43	5	5
Github	128 144	176 144	43x43	5	5
Gitlab	128 190	176 190	43x43	3	5
Telegram	127 236	176 236	45x45	x	4

all issues fixed in this PR:

• center gitub icon
• telegram / discord icon resize to 43x43
• consistent spacing between icons (5 pixels below/between)

so we now have 43+5 actual spacing to avoid confusing the next generation

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[plowsof]

appears to be fine: https://discord.gg/SyGUMWBqvF (just not working for me as ive not verified my discord account)

[nahuhh]

https://discord.com/invite/SyGUMWBqvF

does this one work?

[HardenedSteel]

can we just split the icons? which would be easier to work in future

[nahuhh]

can we just split the icons? which would be easier to work in future

We were already using a (broken) sprite sheet. This PR has 2 commits.

1. fixes the broken sheet
2. pulls in discord commit

IMO, if we want to change the sprite to individual icons, that is a new PR

[plowsof]

sprites have pros/cons. reducing network requests to getmonero by allowing clients to used the cached image seems beneficial enough to just fix/set and forget as we don't work with them often/ever. a PR adding new separate icons would only be warranted if it came with a redesign or quality increase without any increase in network load to the server using fancy new tech (im not qualified enough to produce such a PR of course)

• a several second search returns "lazy images" for example, are we using them already? i don't know https://imagekit.io/blog/lazy-loading-images-complete-guide/

*theres a plugin for that^ works great.

[plowsof]

ignore my above comment about lazy-loading, since 2020 its a default in most/all browsers. the fact that we have a sprite image on our website is embarrassing. we have separate icon files for other things so this is just a relic of the past and needs to be deprecated / replaced with separate, updated icons (which involves design work + style edits to display correctly), but, as @nahuhh mentions above, this PR is fixing the current broken icons/coordinates and pulling in a new discord icon + reducing the original sprite image size by 29% after the work was already done and reviewed so i support this merge. from this point on, no more edits to sprites!

[nahuhh]

also:

- Maintainers SHALL merge correct patches from other Contributors rapidly.
- Any Contributor who has value judgments on a correct patch SHOULD express these via their own patches.