[ServerManage] Add more input validation

[Injabie3]

When saving icon/banner images, we should validate some other things:

• See if image is below a certain file size (discord imposes a limit, should check what that is).
• Minimum dimensions as per recommended.

Also should add try/except block to background loop, since we don't ever want this loop to fail. Should use logger to print traceback as well.

[quachtridat]

I have gathered information from Discord and some other sites.

Image upload size restrictions

• Default: 8 MB
• Nitro Classic: 50 MB
• Nitro: 100 MB
• Server Boost: 50 MB at level 2 for all members, 100 MB at level 3 for all members.

Dimensions

Recommended for server banners: 960 x 540 pixels
Recommended for server icons: 512 x 512 pixels

References

Discord: https://support.discord.com/hc/en-us/articles/360028038352
Discord: https://support.discord.com/hc/en-us/articles/360028716472
Pixelied: https://pixelied.com/blog/discord-profile-picture-size
How-To Geek: https://www.howtogeek.com/667816