freetrader authored Apr 11, 2021 and Calin Culianu committed Apr 11, 2021

Co-authored-by: Andrew Stone <g.andrew.stone@gmail.com>

This is a partial backport of BCHUnlimited changes from GitLab MR 2401

bitcoinunlimited/BCHUnlimited!2401

Summary

Enforces validation of the transaction viewer setting in Settings ->
Options... -> Display such that only valid URLs starting with `http`
or `https` are allowed. Hosts for existing invalid URLs in GUI
configuration are also not displayed in the context menu anymore
(blocked at startup).

Also add a unit test for the URL validation function.

Test plan:

- `ninja check-bitcoin-qt`, check there are no Qt GUI unit test failures
- Enter a URL that is invalid or does not start with http:// or https://
  for the txviewer, and observe that it is refused with an error message:
  "Not a valid HTTP or HTTPS URL"
- Enter a valid URL and restart the application, check that the host set
  in the configuration is displayed in the context menu in Transactions tab
- Stop the application, and modify the `strThirdPartyTxUrls` configuration
  item in the `[General]` section of the appropriate GUI configuration file
  for the network on which you're testing. On Linux, these config files are
  typically stored under `$HOME/.config/BitcoinCashNode/`.
  Set this item to an invalid URL, e.g. one starting with `ftp://` .
- Restart the application and verify that the host of that URL is no longer
  displayed in the context menu.
- Stop the GUI, correct the URL once more in the GUI config file, and restart.
  Verify that the host re-appears in the context menu.