Integrate CKB Explorer with Nexus

[Keith-CY]

This idea is from https://github.com/orgs/ckb-js/projects/5?query=is%3Aopen+sort%3Aupdated-desc&pane=issue&itemId=24601839

and Magickbase/shaping#29, a portfolio tracker for CKB ecosystem.

Nexus team is going to implement Nervo DAO web in Nexus Demo but is bothered by the scarce resource. Meanwhile, there was a project to provide aggregated data based on cell models.

Then this issue comes. If we can deliver the Nervos DAO web inside CKB explorer first, then the aggregated data.

With the addresses exported from Nexus, CKB Explorer can provide data at the account level instead of the address level, which is more meaningful to users of Nexus, Neuron, and other HD wallets.

[Keith-CY]

HD Account view: Magickbase/neuron-public-issues#145

[Keith-CY]

PRD of assets view: https://vs0cjf.axshare.com/#id=y4tbx2&p=_253_integrate_ckb_explorer_with_nexus&g=1

[Sven-TBD]

I've made some adjustments to the original Integration:

1. Add a connect-wallet window on which we can choose to connect by Nexus or WalletConnect
2. Delete some status of portfolio btn. specifically, I replace the portfolio btn with a connect wallet button.
3. WalletConnect could be supportive as well.

FYI, Explorer Prototype: https://vs0cjf.axshare.com/#id=mnuqvu&p=explorersupportwalletconnect&g=1
And accordingly, the WallectConnect proposal is here: Magickbase/neuron-public-issues#144 (comment)

cc @Kirl70