Add last checked timestamp for every monitor

[rathorevaibhav]

Describe the task
In the card for each monitor, we can have the following things added:

1. A text that says "5min ago" or something similar based on the last checked timestamp
2. On hover on this, it will show the exact time when 5m ago was (like GitHub does)

[rathorevaibhav]

Done in PR #68 and #81