Add KeepBranchesOnMailMissionCompletion

[Fayti1703]

This fixes a base-game issue where completing a branch mission via a MailServer erroneously clears out the branch missions of the next mission.

The short of it is: we delete these lines from Hacknet.MailServer->doRespondDisplay:

          for (int index = 0; index < this.os.branchMissions.Count && !flag; ++index)
          {
            flag = this.attemptCompleteMission(this.os.branchMissions[index]);
            // ------------------------ >8 ------------------------
            if (flag)
              this.os.branchMissions.Clear();
            // ------------------------ >8 ------------------------
          }

Note that if attemptCompleteMission returns true, the OS's activeMission and branchMissions have already been set to the values for the next mission by the mission completion code.

---

Compatibility impact

The changes from this PR fix an issue with the base game's "ThemeHack"/"Naix" missions.

Extension behavior may also be altered, though I'm not aware of any Extensions intentionally using this behavior. Conversely, I am aware of one in-development Extension that includes a workaround for this issue.