Analytics and statistics for bridge operation time

[polymorpher]

We should add analytics and collect statistics on completion time for each bridge operation (for all chains and assets). The statistics should be shown to the user at landing page, including at least average, median, 5%, 95%. This can be visualized using percentile-bars similar to what people use for stock price charts.

The benefit is clear:

1. Users have a realistic expectation of how long they should wait
2. Users can detect when something goes wrong (since time takes too long) so the amount of support ticket would be reduced
3. Users can identify which source chain is faster (such as L2), therefore be encouraged to use them instead of slower sources such as Ethereum
4. Users may feel the bridge is more reliable since they can see actual data and its reliability at first glance
5. Users and developers can track these statistics over-time and identify temporary issues / measure improvements