Explorer HTML v0.8: Handle multiple networks and explorer nodes (#2012)

* Skip missing blocks (for pBFT chains)

- Manage a new "adj" node parameter, for missing blocks heights in pBFT chains.

* Allow network and node changes

- Allow switching between networks (L1, dappnet, etc.).
- Add network option in URL parameters.
- Add popup to select preferred explorer node.

* Reorganize code

- Some small adjustment to code and comments.
- add <script> tags to separate groups of JS functions and ease navigation through code.
- Add info in the "About" section.
- Add "network" parameter in all links of the "Historical Blocks" page.

* Adjust version number to v0.8

Author: dbadol