Add health indicator for each asset

[shrnkld]

Scenario:

XLite currently displays various assets without any indication of their "health" status. We want to improve this by adding a check for each asset and displaying an indicator based on the result of this check.

Suggested solution:

• Use the getBlockHash(height) method from the rpc-controller.js or modules_rpc-controller.js file to check the blockchain height for each asset periodically. This method returns the hash of a block at a given height.
• Make this check every 30 seconds for each asset.
• If an asset is online (a non-null hash is returned), no additional indicator is necessary.
• If an asset is offline (an empty hash or null is returned), display an indicator next to the asset.
• This indicator should include a message stating "This asset is currently offline!", and a warning icon (design and assets to follow)
• The health check and the indicator should be added to the asset rendering section of the code (possibly in the balance.js component)
• add a method to handle any errors or exceptions in the health check.

QA

• A health check should be performed for each asset every 30 seconds.
• If an asset is offline, an indicator with an appropriate message and icon is displayed next to the asset.
• If an asset is online, no additional indicator is displayed.
• The application should handle any exceptions or errors in the health check and prevent any crash or major disruption.

NB: we should ensure that the frequency of the health checks do not negatively impact the application's performance. If necessary, we will adjust the check interval or implement a method to stagger the checks for different assets (eg only checking when a user views an asset).