set strings for "reaction summaries"

[r10s]

once we have core updated:

• set the stock strings DC_STR_YOU_REACTED and DC_STR_REACTED_BY to reaction_by_you resp. reaction_by_other (may require core update or using core-main, the strings are already on transifex)

• listening to DC_EVENT_REACTIONS_CHANGED for chatlist updates is already done

see deltachat/deltachat-core-rust#5387 for details, screenshot and reasoning