Bugs found during α

[madhead]

• /lang shouldn't have any state
• /expense should check user ids on actions
• /expense should not allow zeros or empty list of participants
• Non-participants should not be allowed to create transactions
• Currencies must be limited to 120-128 chars (current DB limit)
• There should be a fallback in case the user leves the chat after tracking some transactions
• When a new user is added to the group AFTER everybody else registered it could break the bot:

  java.util.NoSuchElementException: Collection contains no element matching the predicate.
      at me.madhead.tyzenhaus.core.telegram.updates.expense.DebtsCommandUpdateProcessor$process$debtsMessage$1.invoke(DebtsCommandUpdateProcessor.kt:114) ~[tyzenhaus-core-0.0.1.jar:?]
      at me.madhead.tyzenhaus.core.telegram.updates.expense.DebtsCommandUpdateProcessor$process$debtsMessage$1.invoke(DebtsCommandUpdateProcessor.kt:88) ~[tyzenhaus-core-0.0.1.jar:?]
      at kotlin.text.StringsKt__AppendableKt.appendElement(Appendable.kt:85) ~[kotlin-stdlib-1.5.10.jar:1.5.10-release-890 (1.5.10)]
      at kotlin.collections.CollectionsKt___CollectionsKt.joinTo(_Collections.kt:3344) ~[kotlin-stdlib-1.5.10.jar:1.5.10-release-890 (1.5.10)]
      at kotlin.collections.CollectionsKt___CollectionsKt.joinToString(_Collections.kt:3361) ~[kotlin-stdlib-1.5.10.jar:1.5.10-release-890 (1.5.10)]
      at kotlin.collections.CollectionsKt___CollectionsKt.joinToString$default(_Collections.kt:3360) ~[kotlin-stdlib-1.5.10.jar:1.5.10-release-890 (1.5.10)]
      at me.madhead.tyzenhaus.core.telegram.updates.expense.DebtsCommandUpdateProcessor.process(DebtsCommandUpdateProcessor.kt:85) ~[tyzenhaus-core-0.0.1.jar:?]
      at me.madhead.tyzenhaus.core.telegram.updates.expense.DebtsCommandUpdateProcessor$process$1.invokeSuspend(DebtsCommandUpdateProcessor.kt) ~[tyzenhaus-core-0.0.1.jar:?]
      at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33) [kotlin-stdlib-1.5.10.jar:1.5.10-release-890 (1.5.10)]
  

• confirmation in /expense should have "Cancel" option
• it shouldn't be allowed to select participants in another user's transaction