Update Legistar to support backfill (and other filter types?)

[zstumgoren]

LegistarSite.scrape (and possibly others) does not support the ability to configure downloading of older agendas and minutes, along with other standard behaviors illustrated in CivicPlusSite.scrape. Specifically:

• Ability to filter by start and end date
• Ability to limit downloads by asset type
• Ability to limit downloads by file size

LegistarSite.scrape currently hard codes the date to begin scraping to 2021. We should update the function signature to accept start_date and end_date parameters. These parameters should:

• Have the current day set as default for both params
• set the since parameter based on the start_date

Some additional changes that would be helpful for standardization purposes:

• Set download=False by default
• Apply asset type filters
• Apply file size filters

[zstumgoren]

@fgregg I updated the ticket body to more broadly reflect some other possible standardizations as well. Date-based filtering is most important, but it would be great to match the functionalities on CivicPlusSite.scrape as closely as possible