[LDN-2023] Make recent changes requested

[chriscoffee]

There's a few changes here so I've bullet pointed them below:

• Snyk have paid and upgraded to Silver
• WMPromus have paid as Silver. (new sponsor so added ./data and ./static)
• Added three community sponsors for local user groups
• Updated DOXLON image to new one they sent over and backdated previous image.

$ grep -r "doxlon" ./data
./data/sponsors/doxlon-before-20230821.yml:name: "doxlon"
./data/sponsors/doxlon.yml:name: "doxlon"
./data/events/2020-london.yml:  - id: doxlon-before-20230821
./data/events/2018-london.yml:  - id: doxlon-before-20230821
./data/events/2023-london.yml:  - id: doxlon
./data/events/2019-london.yml:  - id: doxlon-before-20230821

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[phrawzty]

Thank you @chriscoffee for the excellent PR description. It's really helpful!