Reduce Webkit rendering failures

[shruti0085]

Description of changes:
This change is being made in correspondence to a mynah change aws/mynah-ui#429
Certain icons were problematic when rendered by Webkit used by Eclipse on Mac which led to errors. Error was caused due to hitting transparency limits: WebCore::GraphicsContextCG::endTransparencyLayer().

• To mitigate it in addition to upstream change, eclipse client will now inject a flag to the backing UI JS to allow it to skip styling that handles transparency layers in particular within chat-item-cards.
• In addition the progress.svg icon shown when a chat response is in progress is a complex animation that overloads Webkit. To mitigate it, a simple static spinner icon now overrides that problematic icon on the Eclipse side.
• The text rate limiting to avoid flooding the Webview has been reduced to prevent delays and the lastProcessingTime computation has been fixed. This was accidentally delaying (additionalMessage)button state changes as well that are not rate limited.

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