Simplify search scopes and fix search on error pages

[lukaw3d]

Includes #1129 and #1157
Supersedes #1145 and #1146

I'm pretty sure this behaves the same way. @csillag @buberdds please check.
Do we have a list of edge-case URLs we can put into E2E tests?

• scope = undefined
• scope = undefined but isn't an error (https://explorer.dev.oasis.io/search?q=abc)
• scope.valid = false
• specify network but not layer

Before	#1145 and #1146	After
https://explorer.dev.oasis.io/abc/xyz (broken)	https://9855550d.oasis-explorer.pages.dev/abc/xyz	https://2f68a07b.oasis-explorer.pages.dev/abc/xyz
https://explorer.dev.oasis.io/abc/xyz/block (broken)	https://9855550d.oasis-explorer.pages.dev/abc/xyz/block	https://2f68a07b.oasis-explorer.pages.dev/abc/xyz/block
https://explorer.dev.oasis.io/abc/xyz/search?q=123 (broken)	https://9855550d.oasis-explorer.pages.dev/abc/xyz/search?q=123	https://2f68a07b.oasis-explorer.pages.dev/abc/xyz/search?q=123
https://explorer.dev.oasis.io/testnet/xyz (search broken)	https://9855550d.oasis-explorer.pages.dev/testnet/xyz	https://2f68a07b.oasis-explorer.pages.dev/testnet/xyz
https://explorer.dev.oasis.io/testnet/xyz/block (search broken)	https://9855550d.oasis-explorer.pages.dev/testnet/xyz/block	https://2f68a07b.oasis-explorer.pages.dev/testnet/xyz/block
https://explorer.dev.oasis.io/testnet/sapphire	https://9855550d.oasis-explorer.pages.dev/testnet/sapphire	https://2f68a07b.oasis-explorer.pages.dev/testnet/sapphire

[github-actions]

Deployed to Cloudflare Pages

Latest commit:	723d21a6185f039a9b968d42457168e9f37bcb74
Status:	✅ Deploy successful!
Preview URL:	https://7e5602ec.oasis-explorer.pages.dev

[csillag]

I have some relevant unpublished code, with the following goals:

• Make it obvious that the scope we get from the URL might be invalid. (The valid flag is not there.)
• Make a distinction between UncheckedScope (which might yet turn our to be invalid) and Scope (which must be valid) at the input of various components / functions.
• Therefore, make in mandatory (on the typescript level) to apply the appropriate checks when we want to pass an unchecked scope to a component that can only handle a valid one. (This requires some code changes at multiple points int he codebase.)

I think this provides the ultimate protection against components being surprised by illegal data.

Unfortunately my latest implementation (according to the above plan) is still uncommited.

@lukaw3d I haven't checked your implementation yet, does it set the same goals?