Implement planet activity logic

[lanedirt]

Implement planet activity logic indicator that shows on the galaxy and espionage report interface whether the planet has seen activity in the last 60 minutes or not.

Activity is determined based on:

• When the player is logged in and switches to a planet the planet is automatically "updated". Updated means the production is calculated and resources are added to the planet. Any finished buildings from the build queue are added to the planet etc.
• When this update happens the planet record field updated_at in the database gets updated to the current time. This field can be used to determine the activity.
• When a foreign player sends an espionage probe with espionage mission to another planet, upon arrival, that planet gets updated and then it is deemed "active" as well.

Implement this activity indicator logic in the following places:

• [ ] Galaxy overview
• [ ] Espionage message

See example of what this indicator should look like in the game:

• Red triangle icon means: activity in last 15 minutes
• Orange square with number in it: amount of minutes (when > 15 minutes)
• Nothing: last activity was more than 60 minutes ago.

Espionage mission message:

[lanedirt]

Some work is already being done in #265. Espionage and attack missions now trigger a planet update. It updates the date column used for the planet resource update. This field can then also be used to determine the actiivty.

[Jbaukens]

/assign