New custom seed column of the plan

[wlandau]

Prework

• Read and abide by drake's code of conduct.
• Search for duplicates among the existing issues, both open and closed.

Description

Supports #945 (see later in the thread).

• plan$seed should supersede drake's automatic seeds, even when a global seed is set in make().
• We should also cast the seed to an integer before storing it.
• Show the seed in drake_history().
• We need a new seed trigger, e.g. trigger(seed = TRUE).
• deps_profile() should show the seeds.
• Allow users to change the global seed since now it simply invalidates targets.

[wlandau]

Fixed via #949