Almost-adjacent committee memberships should keep 'member since' date

One-sentence description

On a committee page, the 'member since' date should ignore small gaps of 1 day in membership history, since these are typically administration errors.

See e.g. @lscholten's Technicie membership.

Current behaviour

When a member has a 1-day gap (member from t to t+x, chair from t+x+1 to now), the join date is reset to the new membership's starting date (i.e. t+x+1).

Expected behaviour

When a member has a 1-day gap (member from t to t+x, chair from t+x+1 to now), the join date should be the original join date (i.e. t).

Steps to reproduce

1. Create a membership from t to t+2years
2. Create a membership from t+2years+1day to now
3. Observe that 'committee member since' says 't+2years', not 't'